Ultra-thin flexible inductor

ABSTRACT

SMT-components known in the art usually have a thickness of approximately 1 mm and no flexibility. According to the present invention windings for an inductor are realized within a substrate, preferably by using copper layers which are already in the substrate. Then, thin metal sheet layers of high permeable material are laminated on top and bottom of the substrate. These layers are structured and then form the magnetic core of the inductor. Advantageously, an inductor may be provided with a very small building height.

The present invention relates to an inductor and to a method formanufacturing an inductor.

In a great number of today's electrical devices, such as mobilecommunication devices, voltages are required that differ from aDC-voltage provided for example from a battery. To convert the voltageefficiently, inductors are needed. Today, thin surface mounted (SMT)inductors are used. They are offered by various manufacturers. Such atypical SMT inductor comprises a thin drum made of sintered ferrite. Thediameter of the core may be approximately 4.3 mm and the height of thecore may be approximately 1 mm. A winding is formed by thin copper wirewound between upper parts and a lower part of the core. Such SMTinductors are usually provided with plastic fixtures with contacts tomount the device to a printed circuit board (PCB).

Due to the fact that a plastic fixture usually needs to be provided andthat the core needs to be specially shaped with a gap of a large aspectratio to accommodate the wire winding, such SMT inductors arecomplicated to manufacture and rather expensive. In addition to that,due to the additional plastic fixture, a building height of the SMTinductor in the range of 1 mm which is too large for an application inspace sensitive applications such as, for example, mobile phones.

It is an object of the present invention to provide an inductor with areduced thickness.

According to exemplary embodiment of the present invention the aboveobject may be solved with an inductor as set forth in claim 1,comprising a substrate with a first side and a second side, a windingwhich is embedded in the substrate and a core. The core comprises afirst softmagnetic metal sheet and a second softmagnetic metal sheetwhich are respectively arranged on the first and second sides of thesubstrate such that the winding is at least partially covered by thefirst and second metal sheets.

In other words, according to this exemplary embodiment of the presentinvention, two thin layers of sheet metal are provided on the sides ofthe substrate including the winding. Thus, advantageously an ultra-thininductor with integrated windings may be provided. Furthermore, theinductor, according to this exemplary embodiment of the presentinvention, has a simple design which may be manufactured at reducedcosts. No specially shaped drum core needs to be provided. This makesthe inductor according to the present invention suitable for massmanufacturing. Also, advantageously, this inductor is very reliable dueto the fact that its magnetic core consists of metal sheets provided onthe substrate. Increased reliability may also be provided due to thefact that no soldering connections between the inductor and thesubstrate are necessary.

According to another exemplary embodiment of the present invention asset forth in claim 2, the winding is a structured copper layer in asubstrate. According to this exemplary embodiment of the presentinvention a substrate already containing copper layers such as a PCB maybe used. Thus, the winding may be formed during the same processing stepduring which other circuit structures in the PCB are formed. Thus, thewinding is “for free” due to the fact that when providing the othercircuit structures, the copper material has to be provided in any casein the substrate and the manufacturing process to structure these copperlayers is also necessary in any case.

Furthermore, due to the fact that the winding comprises a structuredcopper layer in the substrate, complex winding layouts may be obtained,for example, by wet chemical etching. Such complex winding layouts may,for example be necessary to manufacture transformers or intermediateconnections. With this, circuit typologies may be possible, whereon theone compiled with a complex winding is used instead of two or moresimple inductors. Due to this, advantageously, the component count maybe reduced and the size of a circuit comprising such components may bereduced further.

According to another exemplary embodiment of the present invention asset forth in claim 3, the metal sheets are laminated to the first andsecond sides of the substrate. Due to the lamination of the metal sheetsto the substrate, an inductor may be provided which is an integral partof the substrate or PCB. Due to the lamination of the metal sheets tothe substrate, an inductor may be provided with an improved reliability.Also, due to the lamination, no soldering interconnections are necessarywhich further improves the reliability and reduces the costs ofmanufacturing.

According to another exemplary embodiment of the present invention asset forth in claim 4, the metal sheets are made of a high permeablemetal such as μ-metal, amorphous metal or nanocristaline metal. Due tothe fact that these high permeable metal sheets are available with apermeability which is larger than 10.000, which is ten times more thantypical ceramic ferrite, and which saturation flux density is about fivetimes higher than that of typical ceramic ferrites, the magnetic core,i.e. the metal sheets, may be very thin such that an inductor having areduced thickness may be provided.

According to another exemplary embodiment of the present invention asset forth in claim 5, the core plates, i.e. the metal sheets, are soclose together such that a distance between them can be considered as anair gap in the magnetic path occurring in the core during operation ofthe inductor. Hence, according to this exemplary embodiment of thepresent invention preferably thin substrates having a thickness of lessthan 1.2 mm or less than 1 mm are applied.

According to another exemplary embodiment of the present invention asset forth in claim 6, the substrate is a flexible substrate. Due tothis, and due to the fact that the metal sheets are not brittle as, forexample, sintered ferrites, a bendable and a flexible inductor may beprovided. Furthermore, the small thickness of the magnetic core,according to this exemplary embodiment of the present invention, adds tothe flexibility of the inductor.

According to another exemplary embodiment of the present invention asset forth in claim 7, slits are provided in the metal sheets.Advantageously, these slits are arranged such that eddy currentsoccurring in the metal sheets during operation of the inductor areminimized or prevented. According to an aspect of this exemplaryembodiment of the present invention, the slits are arrangedperpendicular to the possible eddy current flow direction. Since theused eddy currents usually flow substantially perpendicular to themagnetic field, according to another aspect of this exemplary embodimentof the present invention the slits are arranged substantially parallelto the direction of the magnetic flux occurring in the inductor duringoperation. Due to this, possible eddy currents are minimized while theslits only have a very small impact on the magnetic flux. In case acircular inductor is provided, according to another aspect of thisexemplary embodiment of the present invention, due to the fact that themagnetic flux is radially orientated in the circular inductor, the slitsare arranged in radial direction.

According to another exemplary embodiment of the present invention asset forth in claim 8, the number of slits increases radially to theoutside of the magnetic core, i.e. to the outside of circular metalsheets. Advantageously, these slits are very narrow.

According to another exemplary embodiment of the present invention asset forth in claim 9, further slits are provided in the metal sheetswhich are perpendicular to the direction of the magnetic flux occurringin the core during operation of the inductor. Advantageously, theseslits may allow to lower the magnetic flux and also the inductivity ofthe inductor. This may be advantageous in application where it isnecessary to prevent the inductor from saturation.

According to another exemplary embodiment of the present invention asset forth in claim 10, a multilayer inductor is provided on a pluralityof sheet metals that are respectively stacked on each other.Advantageously this may allow for inductors with a higher magnetic fluxin the core. Advantageously, such multilayer inductors according to thisexemplary embodiment of the present invention may be manufactured at lowcosts.

According to another exemplary embodiment of the present invention asset forth in claim 11, the width of the slits provided in the metalsheets is varied in the different layers of metal sheets stacked on eachother. According to an aspect of this exemplary embodiment of thepresent invention, the inner layers are provided with larger slits thanthe outer layers which, advantageously, may allow for a homogeneous fluxdistribution in the core.

According to another exemplary embodiment of the present invention asset forth in claim 12, a method of manufacturing an inductor is providedwhere the first and the second metal sheets are laminated on sides of asubstrate comprising a winding embedded in the substrate.Advantageously, according to this exemplary embodiment of the presentinvention a very simple manufacturing method is provided formanufacturing an ultra-thin inductor. Claims 13 and 14 provide forfurther exemplary embodiments of the method according to an exemplaryembodiment of the present invention as set forth in claim 12.

It may be seen as a gist of an exemplary embodiment of the presentinvention that an inductor is provided comprising a substrate with anembedded winding and metal sheets arranged on both sides of thesubstrate forming the core of the inductor. According to an aspect, thewinding of the inductors made by copper tracks in the substrate whichmay be a PCB or a flex foil. Thus, the core may be made by thin highpermeable metal sheets which may be structured and laminated to thesubstrate. Advantageously, this may allow to reduce a building height ofthe inductor while not enlarging a footprint area compared to knownsolutions. Furthermore, the costs for manufacturing those inductors maybe reduced.

These and other aspects of the present invention will become apparentfrom and elucidated with reference to the embodiments describedhereinafter.

Exemplary embodiments of the present invention will be described in thefollowing, with reference to the following drawings:

FIG. 1 shows a sectional view of an inductor according to the firstexemplary embodiment of the present invention.

FIG. 2 shows an exemplary embodiment of a winding layout as it may beused in the inductor of FIG. 1.

FIG. 3 shows another exemplary embodiment of a winding layout as it maybe used in the inductor of FIG. 1.

FIG. 4 is a top view of an exemplary embodiment of a magnetic corelayer, i.e. metal sheet with radial slits and the underlying winding ina top copper layer according to an exemplary embodiment of the presentinvention.

FIG. 5 shows sectional views of different manufacturing states of asecond exemplary embodiment of an inductor according to the presentinvention.

FIG. 6 shows sectional views of manufacturing states of a thirdexemplary embodiment of an inductor according to the present invention.

FIG. 7 shows a sectional view of a fourth exemplary embodiment of aninductor according to the present invention.

FIG. 8 shows a sectional view of a fifth exemplary embodiment of aninductor according to the present invention.

FIG. 9 shows a flex foil with a winding according to an exemplaryembodiment of the present invention as it may be used for example in theinductors of FIGS. 1, 5, 6, 7 and 8.

FIG. 10 shows a circular metal sheet as is may be used for the core ofthe inductors depicted in FIGS. 1, 5, 6, 7 and 8.

FIG. 11 shows the flex foil with winding a FIG. 9 and the flex inductorof FIG. 10 arranged on a flexible substrate according to an exemplaryembodiment of the present invention.

FIG. 1 shows a sectional view of a first exemplary embodiment of aninductor according to the present invention comprising a substrate 2with a first side and second side. Within the substrate 2 there isprovided a winding 6 and 8. The winding 6, 8 is embedded in thesubstrate 2 and thus forms an integral part of the substrate 2. A coreof the inductor is formed by softmagnetic metal sheets arranged on thefirst and second sides of the substrate 2 such that the winding 6, 8 isat least partially covered by the metal sheets 4. The softmagnetic metalsheets 4 arranged on the substrate 2 have a circular shape. Thethickness of the metal sheets 4 may be very thin such as in the range of25 μm to 100 μm. However, it is also possible to use metal sheets havinga thickness in the range of 50 μm to 150 μm or 15 μm to 75 μm. The metalsheets may be made of a high permeable metal material with apermeability which may be larger than 10.000. Such permeability is tentimes higher than that of typical ceramic ferrites. Furthermore, a fluxsaturation of this material, according to an aspect of this exemplaryembodiment of the present invention, is approximately five times higherthan that of ferrites. Advantageously, due to this the magnetic core,i.e. the metal sheets 4 may be made much thinner in comparison tomagnetic cores made of sintered ferrites. According to an aspect of thisexemplary embodiment of the present invention, the metal sheets 4 aremade of a material selected from a group consisting of μ-metal,nanocristaline metal and amorphous metals. All three materials areavailable from the Vakuumschmelze in Hanau, Germany. As amorphous metal,for example VitroVac may be used which is also available fromVakuumschmelze in Hanau, Germany.

Advantageously, μ-metal is the most well known type. It has only mediumhysteresis losses. VitroVac has even lower hysteresis losses andnanocristaline metal has the lowest hysteresis losses of the materialsnamed above and may, therefore, be the material selected for a preferredembodiment of the present invention. From Vakuumschmelze, thesematerials are available as metal sheets with a thickness varying from 25μm to 50 μm and up to several 100 μm.

Advantageously, due to the fact that the windings are embedded in thesubstrate 2, which itself is used as a part of the component, i.e. theinductor, a total building height 14 of the inductor may be reducedsignificantly in comparison to the conventional SMT component. Forexample, a total building height 14 of less than a millimeter may beachieved. Even lower building heights with less than 200 μm arefeasible.

According to an aspect of this exemplary embodiment of the presentinvention, the inductor depicted in FIG. 1 has the same length and widthas for example a conventional 10 μH SMT inductor. Therefore, theintegrated inductor according to this exemplary embodiment of thepresent invention may immediately be used to replace SMT inductor in thesame area. Since as already mentioned above, the thickness of the coresheets 4 may be as low as 0.025 mm, the total thickness may be reducedto 200 μm or even less.

Furthermore, as may be taken from FIG. 1, two inductor layers 6 and 8are provided which are preferably realized as copper layers.

Winding layouts which may be used for the windings 6, 8 are depicted inFIGS. 2 and 3 and will be described in the following.

Reference numeral 10 in FIG. 1 designates copper tracks which may beused for interconnections.

A thickness of the substrate 2 and by that a distance of the metalsheets 4 laminated onto the sides of the substrate 2 may be selectedsuch that the distance between the metal sheets can be considered as anair gap in the magnetic path occurring in the magnetic path duringoperation of the inductor. Thus, preferably a thin substrate 2 is used,such as for example flex foils, which allows these inductors not to havea too large “air gap” i.e. distance between the metal sheets 4.

Due to the fact that, according to an aspect of this exemplaryembodiment of the present invention, the substrate 2 is a flexiblesubstrate, such as a flex foil, and due to the fact that metal sheets 4are used and not sintered ferrites as known in the art, a bendable andflexible inductor may be provided. The flexibility is furthermoreimproved by the fact that the magnetic core, i.e. the metal sheets 4have a very small thickness. Thus, advantageously, the flexible inductoraccording to an aspect of this exemplary embodiment of the presentinvention, may be used for example for variable electronics such as inclothes, in electronics for healthcare, in flexible displays or bendablelamps, e.g. for automotive applications. Furthermore, such flex foilinductors with integrated winding and flexible magnet core may be inparticular advantageous for mobile phone display circuits.

Reference numeral 12 in FIG. 1 designates slits in the upper metal sheet4. According to an aspect of this exemplary embodiment of the presentinvention, such slits 12 are provided in both metal sheets 4 forming thecore of the inductor. However, it is also possible to provide theseslits 12 only in one metal sheet 4 of the inductor.

The slits 12 are provided to overcome a potential disadvantage of thehigh permeable metal sheets 4 which is that the material of these sheetsis highly conducting. This may cause an induction of large eddy currentsin the sheets 4. These eddy currents may cause unwanted losses and mayalso deteriorate the inductive properties of the inductor.

According to an aspect of this exemplary embodiment of the presentinvention, the flowing of such eddy currents will be prevented orreduced due to the introduction of the slits 12 in the metallic magneticcore, i.e. in the metal sheets 4.

The slits 12 may be arranged perpendicular to a direction of the eddycurrent flow occurring during the operation of the inductor or toprevent reduction of the eddy current flow: The reduced eddy currentsflow perpendicular to the magnetic fields in the inductor during theoperation of the inductor. Therefore, preferably, the slits 12 may bearranged in a direction substantially parallel to the direction of themagnetic flux. Due to this, the eddy currents may be minimized while theslits 12 have only a very limited impact on the magnetic flux occurringin the core during operation of the inductor. In a circular inductorsuch the one depicted in FIG. 1, the magnetic flux in the core isradially orientated. Due to this, as depicted in FIG. 1, are arranged inradial directions.

The width of the slits should be as small as technologically possible.As an estimation, the widths of the remaining core segment should besmaller than the penetration depth which describes the fact that highfrequency currents tend to flow on the surface or edges of a conductor.Since the smallest width is also limited due to technologicalconstraints, a layered, such as the one depicted in FIG. 1, may bepreferred where the number of slits increases radially to the outside ofthe magnetic core, i.e. to the outside of the magnetic sheets 4.

As already indicated above, the metal sheets 4 may be laminated to thesubstrate 2 such as to a flex foil. This may be made in the same manneras a lamination of copper layers 2, for example the flex foil. In orderto improve an adhesion of the metal sheets to the surfaces of the flexfoil, the metal sheet may, for example, be silicated on the respectivesurface to be laminated to the substrate 2.

Apart from the above-mentioned application of these inductors, due tothe fact that such inductors may be favorable for the application inlower frequencies such a below 10 MHz, a preferred application of theseinductors may be power converters.

FIGS. 2 and 3 show top view of winding layouts according to an exemplaryembodiment of the present invention, if they may be used for the winding6, 8 (i.e. the copper layers 6, 8) of the inductor depicted in FIG. 1.As may be taken from FIGS. 2 and 3, the windings may have the shape of aspiral. A comparison of the winding direction of views 2 and 3 showsthat, according to an aspect of this exemplary embodiment of the presentinvention, a winding direction of both layers is opposite to each other:In FIG. 2 the winding direction is clockwise while in FIG. 3 the windingdirection is counter-clockwise.

The winding layout depicted in FIGS. 2 and 3 may be used to form a 10 μHinductor realized with two copper layers in a standard enormity with 80μm track width and 80 μm track distance. As depicted in FIG. 1, the twospirals depicted in FIGS. 2 and 3 are arranged in the substrate 2 aboveeach other. They are interconnected to each other by a via betweencontacts 16 and 18. Such winding layout may, for example, be inparticular advantageous for the use in a mobile phone display circuit.The contacts 16 and 18 outside of the spirals may be used for furtherinterconnections for example two copper tracks 10 in FIG. 1.

Due to the fact that the winding layouts are realized in copper layersby, for example, wet chemical etching, photographic processes andsuitable manufacturing processes, complex winding layouts may beobtained, for example transformers. Furthermore, as indicted in FIGS. 2and 3, intermediate connections may be realized for example by means ofvias. With this, circuit arrangements may be possible where only onecomponent with a complex winding is used instead of two or more simpleinductors which advantageously may reduce a component count and a sizeof the circuits.

FIG. 4 shows a top view on a circular metal sheet 20 including slits 22and an underlying winding 24 which is a spiral winding. The sheet 20 isarranged on a substrate 26 and the winding 24 is embedded into thesubstrate 26.

As may be taken from FIG. 4, the slits are arranged such that theyextend radially to the outside of the sheet metal 20. Furthermore, thenumber of slits increases radially to the outside of the magnetic core,i.e. the sheet metal 20 such that the slits 20 have different lengths.

The slits 20 are arranged such that they are perpendicular to thedirection of the eddy current occurring during the operation of theinductor. Since induced eddy currents flow perpendicular to the magneticfields, the slits 22 should be arranged preferably substantiallyparallel to the direction of the magnetic flux which in a circularinductor with a circular metal sheet 20, as depicted in FIG. 4, isradially orientated.

FIG. 5 shows sectional views of different manufacturing states of asecond exemplary embodiment of an inductor according to the presentinvention.

The upper manufacturing state depicted in FIG. 5 shows the substrate 28,namely a flex foil with copper winding 30. The second state shows twohigh permeable metal sheets 32 laminated onto the flex foil with copperwinding. Reference numeral 34 in FIG. 5 designates glue and insulationmaterial which is respectively sandwiched between the flex foil 28 withcopper windings 30 and the high permeable metal sheet 32.

The third manufacturing state depicted in FIG. 5 shows the finalinductor according to the second exemplary embodiment of the presentinvention where the high permeable metal sheet 32 has been structured toform the structured magnetic core 36.

The flex foil with the copper winding 30 depicted at the first state inFIG. 5 may be manufactured in a known manner. For example, by laminatingthe copper tracks forming the copper winding 30 to the flex foil.However, the copper winding may also be formed by a photolithographicprocess and etching.

Then for forming the second manufacturing state, a high permeable metalsheet made for example of μ-metal, nanocristaline metal or amorphousmetal may be laminated to either side of the flex foil 20 with copperwindings 30. Preferably, as depicted in FIG. 4, the lamination causesglue and/or insulation material to be sandwiched between the highpermeable metal sheet 32 and the copper windings. 30.

For structuring the high permeable metal sheet to form the structuredmagnetic core 36, a wet chemical etching may be performed which issuitable for mass manufacturing. With new metal, the same wet etchingcan be performed as with a copper layer. In particular, the samephotolithographic process and the same solvent may be used.

Instead of a wet chemical etching, the structuring of the high permeablemetal sheet 32 may be performed by ways of cutting. The structuring ofthe high permeable metal sheet 32 to form the structured magnetic core36 includes the forming of the slits.

FIG. 6 shows sectional views of manufacturing states of a thirdexemplary embodiment of an inductor according to the present invention.In FIGS. 6 the same reference numerals are used to designate the same orcorresponding elements as in FIG. 5.

It may be taken from FIG. 6, the first manufacturing state depicted inFIG. 6 corresponds to the final manufacturing state depicted in FIG. 5with the exception that in FIG. 6, additional high permeable metalsheets 40 have been laminated by means of glue and lamination material38 onto the final manufacturing state depicted in FIG. 5. Then, in thesecond manufacturing state depicted in FIG. 6, the additional highpermeable metal sheets 40 were structured, for example, by wet chemicaletching to form the structured core 42. As may be taken from this secondmanufacturing state, an inductor is provided having a two-layered core36 and 42.

By adding another additional high permeable metal sheet and bystructuring this additional high permeable metal sheet furtherstructured cores 44 may be added to the inductor such that multilayercores are provided.

Such multilayer cores are in particular advantageous for applicationswhere conductors are needed with a higher magnetic flux in the core. Ahigh magnetic flux in the core may not only be realized by simplyincreasing the thickness of the magnetic core due to the skin effect ofthe magnetic flux according to which the flow occurs only off thesurface of the core layers and according to which the inside of thecores would be field free and thus unused. According to this skineffect, the use of several thin stacked insulated layers of highpermeable sheets allows to significantly increase the magnetic flux inthe core.

As may be taken from FIGS. 5 and 6, the manufacturing of such multilayerinductors is similar to the manufacturing of a more simple one-layerinductor, as shown in the final manufacturing state depicted in FIG. 5with the exception that further elimination steps and structuring stepshave to be carried out.

Alternatively to the manufacturing method suggested by FIG. 6, alllayers may be etched in one step. However, an insulating adhesive has tobe provided between the layer which may be structured with the sameetching solvent that is used to etch the high permeable metal sheetlayers.

FIG. 7 shows a sectional view of an inductor according to a fourthexemplary embodiment of the present invention. As may be taken from FIG.7, winding layers 52 are arranged on a substrate 50 on to whichstructured high permeable metal sheet layers 54 have been laminated toform the magnetic core of the inductor. As may be taken from thesectional side view of FIG. 7, the core, i.e. the structured highpermeable metal sheet layers 54 are provided with slits 56. According toan aspect of this exemplary embodiment of the present invention, theseslits 56 are perpendicular to the magnetic flux direction occurring inthe core. Advantageously, these slits 56 may lower the magnetic flux andalso the inductivity of the inductor according to this exemplaryembodiment of the present invention. This may be advantageous forcertain winding layouts where it may be necessary to prevent theinductor from saturation.

FIG. 8 shows a sectional view of a fifth exemplary embodiment of aninductor according to the present invention. As may be taken from FIG. 8this inductor is a multilayer inductor comprising three core layers 58on each side of two windings layers 60 provided on a substrate 62. Asmay be taken from FIG. 8, the core layers 58 are provided with slits 64.

In some cases it may appear that the magnetic flux is concentrated inthe most inner core layer 58, i.e. the core layers 58 close to thewinding layers 60. This may happen due to the fact that these innerlayers 58 may have a shielding effect with respect to the out layers.This may happen in particular when an insulation provided between thecore layers 58 is too thick.

According to an aspect of this exemplary embodiment of the presentinvention, slits 64 which are perpendicular to the flux direction areformed in the core layers 58. Due to this, the magnetic flux is forcedto also flow into the other more outer layers which may improve the fluxdistribution over the layers. Advantageously, a homogeneous fluxdistribution may be achieved. To further improve the homogeneity, thewidth of the slits may be varied in the different core layers 58. Inparticular, the width of the slits in the inner layers 58 may be largerthan the slit widths of slits 64 in outer layers 58.

FIGS. 9 and 10 show samples of an inductor winding (FIG. 9) and astructured flexible core (FIG. 10). The flex foil with the copperwinding depicted in FIG. 9 was manufactured in accordance with astandard flex foil process. The core (FIG. 10) was made of 25 μmVitroVac high permeable metal foil by laser cutting. The core sheetswere then attached to the flex foil by hand with a tesafilm adhesivetape.

FIG. 11 shows a picture of a flex foil substrate including spiralwindings and a structured core laminated thereon. As may be taken fromFIG. 1, the flexibility of the inductor according to the presentinvention is quite substantial.

1. Inductor, comprising: a flexible substrate having a first side and asecond side; a flexible winding having a first winding layer and asecond winding layer, the first winding layer and the second windinglayer being spaced apart and parallel; and a core; wherein the flexiblewinding is embedded in the flexible substrate; wherein the corecomprises a first flexible metal sheet which is arranged on the firstside of the flexible substrate and a second flexible metal sheet whichis arranged on the second side of the flexible substrate such that theflexible winding is at least partially covered by the first flexiblemetal sheet and the second flexible metal sheet; and wherein the firstand second flexible metal sheets are softmagnetic.
 2. The inductor ofclaim 1, wherein the flexible winding is a structured copper layer inthe flexible substrate.
 3. The inductor of claim 1, wherein the firstand second flexible metal sheets are high permeable metal sheets; andwherein the first and second flexible metal sheets are laminated to thefirst and second sides of the flexible substrate.
 4. The inductor ofclaim 1, wherein the first and second flexible metal sheets are made ofat least one material selected from the group consisting of μ-metal,amorphous metal and nanocristaline metal.
 5. The inductor of claim 1,wherein a thickness of the flexible substrate and therewith a distancebetween the first and second flexible metal sheets is such that thedistance can be considered as an air gap in a magnetic path of the core.6. The inductor of claim 1, wherein the first and second flexible metalsheets have first slits; wherein the first slits are arrangedsubstantially perpendicular to an eddy current flow occurring in thefirst and second flexible metal sheets during operation of the inductorto reduce the eddy current flow.
 7. The inductor of claim 6, whereincopper structures of the flexible winding have the form of a spiral;wherein the first and second flexible metal sheets are circular; andwherein a number of the first slits in the first and second flexiblemetal sheets increases radially.
 8. The inductor of claim 1, wherein thefirst and second flexible metal sheets have second slits; wherein thesecond slits are perpendicular to a direction of a magnetic fluxoccurring the in the first and second flexible metal sheets duringoperation of the inductor.
 9. The inductor of claim 1, wherein aplurality of first and second flexible metal sheets are arranged,wherein the plurality of first flexible metal sheets are arranged oneabove the other and wherein the plurality of second flexible metalsheets are one above the other.
 10. The inductor of claim 9, wherein theplurality of first and second flexible metal sheets are provided withthird slits; wherein the third slits are perpendicular to a direction ofa magnetic flux occurring the in the plurality of first and secondflexible metal sheets during operation of the inductor; wherein innerones of the first and second flexible metal sheets have third slits witha first width and outer one of the first and second flexible metalsheets have slits with a second width; and wherein the first width islarger than the second width.
 11. Inductor, comprising: a substratehaving a first side and a second side; a winding; and a core; whereinthe winding is embedded in the substrate; wherein the core comprises afirst metal sheet which is arranged on the first side of the substrateand a second metal sheet which is arranged on the second side of thesubstrate such that the winding is at least partially covered by thefirst metal sheet and the second metal sheet; wherein the first andsecond metal sheets have first slits; wherein the first slits arearranged substantially perpendicular to an eddy current flow occurringin the first and second metal sheets during operation of the inductor toreduce the eddy current flow; wherein copper structures of the windinghave the form of a spiral; wherein the first and second metal sheets arecircular; wherein a number of the first slits in the first and secondmetal sheets increases radially; and wherein the first and second metalsheets are softmagnetic.
 12. Method of manufacturing an inductor, themethod comprising the steps of: providing a flexible substrate having afirst side and a second side, wherein a flexible winding is embedded inthe flexible substrate, the flexible winding having a first windinglayer and a second winding layer, the first winding layer and the secondwinding layer being spaced apart and parallel; and laminating a firstflexible metal sheet on to the first side of the flexible substrate anda second flexible metal sheet on to the second side of the flexiblesubstrate such that the flexible winding is at least partially coveredby the first flexible metal sheet and the second flexible metal sheet tothereby form a core; wherein the first and second flexible metal sheetsare softmagnetic.
 13. The method of claim 12, further comprising thestep of: structuring copper layers to form the flexible winding in theflexible substrate.
 14. The method of claim 12, further comprising thestep of: providing first slits in the first and second flexible metalsheets; wherein the first slits are made by wet chemical etching orlaser cutting.
 15. Inductor, comprising: a substrate having a first sideand a second side; a winding; and a core; wherein the winding isembedded in the substrate; wherein the core comprises a first metalsheet which is arranged on the first side of the substrate and a secondmetal sheet which is arranged on the second side of the substrate suchthat the winding is at least partially covered by the first metal sheetand the second metal sheet; wherein a plurality of first and secondmetal sheets are arranged, wherein the plurality of first metal sheetsare arranged one above the other; wherein the plurality of second metalsheets are one above the other; wherein the plurality of first andsecond metal sheets are provided with third slits; wherein the thirdslits are perpendicular to a direction of a magnetic flux occurring thein the plurality of first and second metal sheets during operation ofthe inductor; wherein inner ones of the first and second metals sheetshave third slits with a first width and outer one of the first andsecond metal sheets have slits with a second width; wherein the firstwidth is larger than the second width; and wherein the first and secondmetal sheets are softmagnetic.